1. Field of the Invention
The present invention relates to a differential refractive index detector used as a detector in an analysis device such as a liquid chromatograph.
2. Description of Related Art
A differential refractive index detector includes a flow cell, a light receiving element and an optical system. The flow cell has two cells spaced apart by a spacer inclined with respect to an optical axis of a measuring light. A sample solution passes through one cell, and a reference solution passes through the other cell. The light receiving element receives the measuring light refracted by the flow cell. The optical system makes the measuring light pass through a slit to irradiate into the flow cell, and guides the measuring light from the flow cell to the light receiving element, and thus forming a slit image on the light receiving element. And, the differential refractive index detector detects the refractive index variation of the sample solution according to a displacement of the slit image on the light receiving element.
FIG. 3 is a schematic view of an example of the differential refractive index detector. A light emitted from a light source 8 passes through a slit 10 to become a measuring light 12. Next, after passing through a lens 14 disposed in front of a flow cell 16, the measuring light 12 is irradiated on the flow cell 16. The flow cell 16 includes two cells S and R, and the cells S and R are spaced apart by a spacer 18. The cell S includes a liquid inlet 22i and a liquid outlet 22o, and the cell R includes a liquid inlet 24i and a liquid outlet 24o. A reflecting mirror 26 is disposed behind the flow cell 16. After being reflected by the reflecting mirror 26, the measuring light already transmitting through the flow cell 16 will re-transmit through the flow cell 16. The reflected light, from the reflecting mirror 26 and transmitting through the flow cell 16, will images on the light receiving element 30 through the lens 14, so as to generate a slit image. A light receiving surface of the light receiving element 30 is divided into two parts, and the slit image is generated in a manner of crossing each region of the light receiving element 30 that is divided into two parts. A zero glass 28 is disposed on a light path between the lens 14 and the light receiving element 30. The zero glass 28 is used to make the slit image to parallelly move on the light receiving element 30. A pulse motor 32 is used to drive the zero glass 28 to rotate, and the pulse motor 32 rotates according to signals from a control and calculation portion 34. According to the rotating angle at this time, the zero glass 28 makes the slit image to parallelly move on the light receiving element 30. A signal processing circuit 36 executes a signal processes to obtain the refractive index variation according to a detecting signal from the light receiving element 30, and the control and calculation portion 34 is used as the differential refractive index detector to obtain an output value.
The refractive index of a substance greatly depends on the temperature, so the temperature dependence varies comply with different substances. Therefore, as for the differential refractive index detector, practically, before a sample is analyzed, the content fluid in the cell (S) which the sample solution passes through and in the cell (R) which the reference solution passes through must be replaced thoroughly. Particularly, when the composition of the mobile phase is changed, it is necessary to adopt a following flow path that is capable of replacing (purging) the content fluid in the cell R. The flow path connected with the flow cell 16 has the following structure, that is, a structure capable of being switched between the analysis process and the purging operation. More specifically, a following flow path is formed during the analysis process, i.e., the outflow liquid from a column (not shown) enters the cell S from the liquid inlet 22i, and outflows from the liquid outlet 22o. And, a following flow path is formed during the purging operation, i.e., the outflow liquid from the column is from the liquid outlet 22o and flows into the cell R through the liquid inlet 24i, and then outflows from the liquid outlet 24o. Therefore, during the analysis process, the mobile phase used for analysis and the sample solution flow into the cell S, and the mobile phase is stored in the cell R. During the purging operation, the cell S and the cell R are mutually communicated, and the mobile phase used for analysis flows into the cell S and the cell R.
When air bubbles are generated in the cell or the composition of the mobile phase is changed, the purging operation must be performed. If the purging is not sufficient, the output value easily changes as the changing of the temperature, such that the detector cannot sufficiently achieve its functions.
In conventional art, any one of the following three methods is used to perform the purging operation. Firstly, the method used by the operator to perform the purging operation manually is that (1) a liquid supply device connected to the differential refractive index detector is used to supply the liquid for a suitable time period under a suitable condition, and then, assures that the output of the differential refractive index detector becomes stable on the frame of a data processing device. In addition, the method of automatically performing the purging operation is that (2) the purging operation is performed for a preset time, and after the above time period, the purging operation is automatically finished (Patent reference 1); (3) the purging operation is performed with a preset volume, and after the liquid supply device has finished supplying the above volume of liquid, the purging operation is finished.
[Patent reference 1] Japanese Laid-Open Patent Publication 2001-033386.
The above three methods have the following defects. According to the method (1), it needs an operator to execute the operation, and under most cases, the operator may waste the time till the purging process finished. Also, when determining whether the purging is sufficiently performed or not, there is always less information for making judgment, so in most cases, it depends on experiences of the operator. And thus, the standard used for the judgment varies from person to person. According to the method (2), the purging operation is automatically finished after during the specified time, so as to reduce the problems relevant to time of the method (1). However, the flow of the liquid passing through the flow cell or the composition of the purging liquid the purged liquid is different, so the purging degree that has been finished in the specified time may be different. In addition, in the purging operation process, due to certain poor situations such as the liquid used for purging is not sufficient, even if the purging is substantially not performed, the purging operation is finished after the specified time. According to the method (3), after the liquid of the set volume has been delivered, the purging operation is automatically finished, so as to reduce the problems relevant to time. However, it is the same as (2) that the purging degree varies depend upon different conditions, and even under the poor situations, the purging operation is still finished.
FIG. 4 is a schematic view of the relation between the purging degree and the rotating angle of the zero glass. The zero glass 28 is rotated, such that the slit image is generated in a manner of crossing each region of the light receiving element 30 that is divided into two parts. As shown in FIG. 4(a), if the purging operation is sufficiently performed in the flow cell with the temperature maintained to be uniform (e.g., 40° C.), the compositions of the content fluid in the cells S and R become completely the same. However, if the purging operation is not sufficient, the compositions of the content fluid in the cells S and R are different from each other, and the refraction will occur on the interface of the cell S/cell R, such that the slit image is biased from the optical axis (as shown in FIG. 4(b)). And then, the zero glass 28 is rotated to make the slit image to parallelly move on the light receiving element 30, so as to correct the bias of the slit image. Therefore, the slit image is uniformly imaged on the region of the light receiving element 30 divided into two parts (the midmost position). It can be viewed from the appearance that, the purging operation is sufficiently performed.